Size reduction machines of the prior art utilize a frusto-conical shaped screen located in a channel between an input and an output. Such size reduction machines are more particularly described in U.S. Pat. No. 4,759,507. In these machines, various screens and impellers are used depending on the size and type of product that is being processed. The screens have apertures in different sizes and shapes to produce a desired milled product.
Once the desired impeller and screen have been selected and the gap therebetween properly adjusted, particulate material is passed through the machine and the size of the material is reduced. The amount or reduction is dependent on a number of factors, including the size of the apertures of the screen, the angle of the wall of the screen, the gap between the wall of the screen and the impeller. In many cases, the desired particle size cannot be achieved with a single pass. The particulate must first be passed through a machine having a first screen and then passed through a machine having a second screen. In some extreme cases, the particulate must be passed a third or fourth time. If the production facility does not have enough machines, the same machine must be dismantled and a second screen must be installed and calibrated. This process is very time consuming, requiring skilled technicians to properly adjust the gap between the impeller and screen. Further, problems of quality control increases as the number of times the machines are dismantled increases. These problems become particularly acute when the machines are used preparing pharmaceuticals.
There have been a number of machines which have attempted to address this problem by increasing the number of stages on the impeller. Such machines are described in U.S. Pat. Nos. 2,822,846 and 3,249,310. These machines are capable of reducing particulate material into a smaller particulate size but are in essence two size reduction machines stacked one on top of the other and mounted on a common shaft. These machines are unable to fit within the size restrictions demanded in the pharmaceutical industry. Further, these machines are unable to meet the strict quality requirements of the pharmaceutical industry.